1. Technical Field of the Invention
The present invention relates generally to sensing equipment and more particularly to an apparatus and a method of calibrating low pressure sensors.
2. Description of Related Art
Pressure transducers which use strain gauges in a Wheatstone Bridge configuration are well known in the art. Such pressure transducers may be configured to produce an output voltage or output current that is proportional to the pressure being sensed. The transducers will also typically have a specific range where the transducer can be used. For example, a transducer will have a rated cold temperature and a rated hot temperature and the transducer will be tested to ensure that it works properly within the rated temperature range. A similar situation occurs with respect to pressures, as a transducer will be rated and tested to function within a certain range of pressure.
Such pressure transducers may be sensitive to various disturbances, such as temperature changes, which, if uncompensated, will cause errors in the pressure reading. Temperature induced errors may be observed as a change in the output of the transducer as temperature varies with zero pressure applied, or as a change in the difference between full-scale output and zero pressure output as the temperature varies. These errors are known as xe2x80x9cthermal effect on zeroxe2x80x9d (or xe2x80x9cthermal effect on offsetxe2x80x9d) and xe2x80x9cthermal effect on span,xe2x80x9d respectively.
Methods are known in the art to compensate for such errors and typically require an initial characterization of the transducer to define any errors. Typically, at least two points from the output signal of the transducer are recorded as ambient temperature is varied over a predetermined range both with zero pressure applied and with a predetermined amount of pressure applied. The pressure applied is typically, but not necessarily, full-scale pressure, and the output is recorded at the same temperature points with zero pressure and with the applied pressure. Based on the output signals, the uncompensated thermal effects are calculated and used to derive the required amount of compensation.
There are several methods available to provide error compensation in a pressure transducer. One common method is to add resistors in series with the bridge supply voltage, and in series with or parallel to the individual bridge resistors. The resistors are chosen based on the particular thermal properties necessary to negate the observed thermal effects, and their values are calculated based on the uncompensated thermal measurements. Error compensation may also be accomplished by laser trimming resistors or thermistors to force voltage changes at the sensor. Another method, known as digital compensation, uses stored data to generate error-correction signals which are added to or subtracted from the uncompensated output of the bridge.
Error compensation to achieve accurate measurements, however, can be a costly and time-consuming process. Frequently, the process of characterizing the transducer, adding compensation, re-characterizing the transducer, and adjusting the compensation must be repeated several times to obtain the desired accuracy. This can be more difficult with particular transducer designs such as, in micro-machined, very low pressure, silicon sensors or those with full-scale pressures of 1 inch of H2O (250 pascals) or less.
FIG. 1 illustrates the process described above in the form of a flow chart. Initially, the transducer is brought into a testing apparatus where it is tested to characterize the transducer (step 102). For example, the transducer may be tested at zero pressure and room temperature (e.g., 25xc2x0 C.). The temperature may then be varied to determine the output voltage as a function of temperature (step 104). In addition, the transducer may then be tested as to the output of the transducer with respect to changes in pressure. This test may or may not involve the use of an additional testing station. After this characterization step, it is determined if the transducer is operating properly (step 106). For example, the transducer may not be producing the correct output at zero pressure. In that case, the transducer would be adjusted through the use of, for example, laser trimming or the addition resistors to the supply voltage such that the response of the transducer is changed (step 108).
After the adjustment, the above steps are repeated to determine if the transducer is behaving in the desired manner. If the transducer is not properly calibrated to produce the correct output, the adjustment process must be repeated. Once the transducer is brought within specification, the process is ended (step 110), and another transducer is processed.
There are several shortcomings to the above-described process. The process is time consuming, as the transducer may have to be calibrated and re-calibrated several times to calibrate the sensor. In addition the calibration process may not be as precise as desired. For example, in order to properly calibrate a sensor, it may be necessary to adjust the resistance of a resistor to within 0.1 ohms by use of laser trimming. However, that degree of precision may not be possible because the mere measuring of the resistance may cause the resistance to change.
Such errors can be compensated for and there are several known methods for doing so. For example, U.S. Pat. No. 6,023,978, assigned to Honeywell International, discloses a pressure transducer that uses two sensors that are mechanically and electrically cross-coupled with each other such that errors associated with one sensor are compensated or substantially cancelled by errors associated with the other sensor.
However, such a transducer may still suffer from various errors. For example, a transducer is ideally calibrated such that the transducer produces a certain voltage at one pressure extreme (for example, 4.25 volts for maximum pressure) and another voltage at another pressure extreme (for example, 0.25 volts for zero pressure), possibly with a linear response between the extremes. In addition, the output may change with respect to temperature: a sensor in a cold environment may have different outputs for a particular pressure than a sensor in a warm environment at the same pressure. While a transducer such as that described in the ""978 patent may adequately compensate for certain mechanical noise issues and gravity issues, such a transducer does not adequately address all of the issues respecting the calibration of the sensor.
Therefore, there is a need for a method and system for calibrating transducers system that results in a more precise calibration and that can be performed more quickly than traditional methods.
The present invention presents a system and method for meeting those needs. The system includes a circuit that is electrically coupled to a sensor. The circuit may be an application specific integrated circuit (ASIC) that may be programmable such that a desired result may be output by the ASIC based on readings of the sensor.
A method for calibrating a transducer is also disclosed. The transducer includes a sensor coupled to an ASIC. The transducer is coupled to a computer and placed within a testing chamber. The computer is further coupled to the testing chamber. Then various conditions can be set by the computer. The output of the sensor is evaluated to determine if the output is within a certain tolerance of an ideal output. If not, it is then determined if the output can be corrected by the programming of the ASIC. If the output can be corrected, then the ASIC is programmed to produce a desired output. Thereafter, the above steps can be repeated for a variety of different conditions.